Conventional numerical control units are designed to obey a set of pre-defined instructions, and prominent among these are instructions for moving a tool or cutterhead along either a straight path or a circular arc, at a prescribed speed, between two points defined in three rectangular coordinates. Therefore, whenever a different kind of movement is desired, it must be broken down into a succession of straight or circular segments. For each such segment, the control will establish a pattern whereby the cutterhead will first be accelerated to the desired speed of motion, then maintained at a constant speed over the middle portion of its travel, and finally decelerated to a stop over the last portion of the segment.
It is well known to mill circular grooves in the wall of a cylindrical bore, mostly as seats for O-ring gaskets or snap rings, by means of a cutter disk that is orbited or revolved around the internal surface so that the cutter teeth will cut the metal for a desired depth. The numerical control of the tool machine is therefore programmed to move the tool forward along the axis of the bore up to the axial position of the desired groove; then to move the tool along a straight radial path until it bites into the material of the wall for the desired depth of the groove; then to revolve the tool along a circular path to cut the groove; and finally to move the tool back to an axial position along a similar straight radial path, when the revolution around the bore has been completed.
With the above grooving procedure, the groove is generally cut by removing a chip layer having the same thickness as the desired groove depth. The removal of such a thick chip in a single pass subjects the spindle to considerable twisting and bending stresses, which are of a vibratory nature, because of the discontinuous teeth. These stresses have an adverse effect on the finishing quality of the groove and also subject the cutting teeth to a considerable wear, which results in shortened tool life and rough finishing of the groove. If it is desired, particularly in case of a very deep groove, to cut the groove in two or more passes, then the same procedure as above (i.e. approaching path, circular path, receding path) must be carried out repeatedly, with increasing diameters, though with considerable waste of processing time.
Moreover, when the cutter disk approaches the wall, its teeth will abruptly meet both a tangential and a radial resistance from the material of the workpiece, giving rise to both a twisting and a bending stress acting on its stem. Since the stem of the tool has a limited rigidity, it will therefore deflect in a small but non-negligible degree and subsequently relax when the tool starts to follow a path tangential to the wall as it cuts the groove. These fluctuations give rise to bumps or irregularities in the groove, which are compounded with the defects mentioned above.
In order to minimize the above defects, it has been proposed that the cutter disk follows a semicircular rather than a straight path in its approach to the bore wall, so that the tool will attack the material at an acute angle; a similar semicircular path is also followed when the tool is withdrawn. Although this maneuver does lead to a more gradual transition and is easy to implement on a conventional control unit, it only has a moderate favorable influence on the bumps caused by initial contact between the tool and the workpiece, and does not remedy the drawback of the stresses incurred by the cutter disk during the one-pass, deep-cutting machining.